The spinal column is a biomechanical structure composed primarily of ligaments, muscles, vertebrae, and intervertebral disks. The biomechanical functions of the spine include: (1) support of the body, which involves the transfer of the weight and the bending movements of the head, trunk and arms to the pelvis and legs; (2) complex physiological motion between these parts; and (3) protection of the spinal cord and nerve roots.
As the present society ages, it is anticipated that there will be an increase in adverse spinal conditions which are characteristic of aging. For example, with aging comes an increase in spinal stenosis (including, but not limited to, central canal and lateral stenosis), and facet joint degeneration. In addition to spinal stenosis and facet joint degeneration, the incidence of damage to the intervertebral disks is also common.
The primary purpose of the intervertebral disk is to act as a shock absorber. The disk is constructed of an inner gel-like structure, the nucleus pulposus (the nucleus), and an outer rigid structure comprised of collagen fibers, the annulus fibrosus (the annulus). At birth, the disk is 80% water, but the water content gradually diminishes with time, causing the disk to stiffen. With age, disks may degenerate and bulge, thin, herniate, or ossify. Damage to disks also may occur as a result of disease, trauma, or injury to the spine.
Disk damage can have far-reaching consequences. By way of example only, both the cervical and lumbar areas of the human spine are, in a healthy state, normally lordotic such that they are curved convex forward. It is not uncommon that in degenerative conditions of the spine, normal curvature is lost. Loss of normal curvature effectively shortens the spinal canal, and decreases its capacity. Further, the absence or loss of normal curvature of the spine moves the spinal cord to a more anterior position, potentially resulting in compression of the posterior portions of the vertebral bodies and the disks. Loss of normal curvature thus disturbs the overall mechanics of the spine, which may cause cascading degenerative changes throughout the adjacent spinal segments.
The surgical treatment of those degenerative conditions of the spine in which the spinal disks are in various states of collapse commonly involves spinal fusion, that is, the joining together of adjacent vertebrae through an area of shared bone. When the shared bone is in the area previously occupied by the intervertebral disk, the fusion is referred to as an “interbody fusion.” Fusion results in formation of a solid bony mass between adjacent vertebral bodies. The newly formed bony mass can assume a weight-bearing function and thereby relieve mechanical pain caused by an unstable degenerative disk. The bony fusion mass further can prevent long-term disk collapse or additional degenerative changes.
Fusion can be accomplished by interbody bone grafting. Typically, grafting requires penetrating the vertebral endplates, which are made of hard bone, to prepare the target vertebrae. Such preparation exposes the spongy, vascular, cancellous bone. Bone grafts then are positioned to be in contact with the cancellous bone and the blood supply. The direct contact between the natural or synthetic bone fragments, with or without other bone growth-promoting materials such as growth factors, initiates a controlled healing process, which results in production of new bone and healing of the graft to both opposed vertebral surfaces. The final result is a single, continuous segment of bone that is composed of the new bony mass between, and fused with, two contiguous vertebrae. Fusion is expected to have a higher probability of success with more direct and extensive contact between the bone graft-promoting materials and the cancellous bone.
Since fusion takes place over time, the spine can remain unstable until fusion is complete. However, spinal instability may contribute to the failure of the fusion. Therefore, a fusion implant is needed that (1) maximizes the probability of success of bone fusion; (2) provides instant stability to the spine while fusion occurs; and (3) is easily implantable and minimizes trauma to the patient and the possibility of surgical and post-surgical complications.